1. Field of the Invention
The present invention relates to reinforced composites and more particularly to composites reinforced by carbon or graphite that has been treated to reduce corrosion between the reinforcement material and adjacent metals.
2. Description of the Background Art
It is well known that the corrosion performance of carbon fiber/aluminum (or magnesium) metal matrix composites is extremely poor in aqueous chloride environments. For example, the corrosion rate of a carbon fiber/aluminum alloy matrix composite is fifteen times greater than that of the non-reinforced alloy in 3.5 weight percent NaCl solution. The corrosion attack occurs as severe exfoliation and general deterioration near diffusion-bonded areas and at the carbon-aluminum interphase. This deterioration is more prominent near crevices, cracks or voids.
Three possible mechanisms can contribute to the accelerated corrosion of carbon fiber/aluminum metal matrix composites. They are: (1) crevice corrosion at voids formed between the fiber and matrix during fabrication, (2) preferential dissolution of interphase metal carbides which leads to the breakdown of aluminum's passivating oxide layer, and (3) galvanic corrosion at exposed surfaces. Because of the large potential difference between graphite and aluminum (1.2 V as determined from experimental data in seawater), many investigators have suggested that galvanic corrosion is the dominant mechanism.
Several others have attempted to overcome the corrosion susceptibility of carbon fiber/aluminum metal matrix composites. In one approach, the carbon or graphite reinforcement fibers are coated with a metal which reduces the galvanic interaction between the metal matrix and the fibers. In U.S. Pat. No. 4,680,093, a metal, such as nickel, is electroplated onto the surface of a carbon fiber which is then used to reinforce a matrix of a different metal. A similar approach is described in U.S. Pat. No. 3,622,283. In these approaches, where the carbon or graphite fiber is coated with a metal prior to incorporation into a matrix of another metal, a problem is that some of the advantages of the graphite/metal interface are lost. Additionally, typical methods of coating, such as electrodeposition, present contamination problems.
Similar corrosion problems also arise in structures where metallic members, which may be susceptable to galvanic corrosion, are in the presence of graphite or carbon-reinforced composites.